JP2002367610A - Nonaqueous secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous secondary cell with high discharging capacity and excellent impedance property even when the added volume of a conduction supporting agent is reduced. SOLUTION: Surface of positive electrode activator is uniformly coated by a silane compound and a conduction supporting agent, and it is used as a positive electrode of the nonaqueous secondary cell. For example, alkoxysilane like methyltriethoxysilane, methyltrimethoxysilane, dimethyldiethoxysilane, or tetraethoxysilane is used as a silane compound, and carbon black or the like is used as the conduction supporting agent. It is recommended to make the coating volume of the silane compound to the 100 weight portion of the positive electrode is 0.01-5 weight portion at silicon conversion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery having excellent impedance characteristics after storage at a high temperature or after a charge / discharge cycle.

[0002]

2. Description of the Related Art With the miniaturization of electronic devices and the spread of mobile phones, there is an increasing demand for secondary batteries having a high energy density. At present, high-capacity secondary batteries that meet this demand include lithium-containing double oxides such as LiCoO ₂ , LiNiO ₂ or LiMn ₂ O ₄ as a positive electrode active material.
A lithium ion secondary battery using a carbon-based material as a negative electrode active material has been commercialized. This lithium ion secondary battery has a high average driving voltage of 3.6 V, which is about three times the average driving voltage of conventional nickel-cadmium batteries or nickel-metal hydride batteries. Further, since a carbon-based material is used as the negative electrode active material and the moving body involved in charging and discharging is lithium, which is a light metal, a reduction in weight can be expected. from now on,
With the growing demand for portable information terminal equipment, it is expected that the mounting of high capacity and lightweight lithium ion secondary batteries will increase more and more.

A lithium ion secondary battery is different from a conventional non-aqueous secondary battery using lithium metal as a negative electrode. A paste in which the above active material is dispersed in a solution together with a binder and the like is used. Both the positive electrode current collector and the negative electrode current collector are formed with a coating film containing an active material on both surfaces of the current collector to produce positive and negative electrodes. Then, those strip-shaped electrodes are spirally wound through a separator to form an electrode body,
A battery is constructed by inserting it into a battery can.

Since the lithium-containing double oxide used for the positive electrode has poor electron conductivity, electron conductivity is ensured by adding a carbon-based conductive assistant such as carbon black (lithium ion secondary oxide). Battery 2nd edition, Masayuki Yoshio /
Ozawa Akiya edition, Nikkan Kogyo Shimbun).

[0005]

As the capacity of lithium secondary batteries has been increased, attempts have been made to increase the specific surface area of the positive electrode active material and reduce the amount of conductive assistants. However, as the specific surface area of the positive electrode active material increases, the contact area between the active material and the conductive auxiliary increases, so that the impedance of the battery increases unless the amount of the conductive auxiliary is increased. Further, in the conventional mixing and dispersion of the positive electrode active material, the conductive auxiliary agent and the binder, it is difficult to reduce the amount of the conductive auxiliary agent, and in particular, it has not been possible to suppress an increase in impedance after high-temperature storage or after cycling.

An object of the present invention is to provide a non-aqueous secondary battery having excellent impedance characteristics after storage at a high temperature or after a charge / discharge cycle.

[0007]

Means for Solving the Problems The present inventors have conducted various studies to solve these problems, and as a result, by coating the surface of the positive electrode active material with a silane compound and a conductive auxiliary, In comparison with the battery characteristics of a lithium ion secondary battery using a mixture of the positive electrode active material and the conductive additive, it was found that the discharge capacity was improved and the impedance characteristics could be significantly improved.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a method of coating the surface of a positive electrode active material with a silane compound and a conductive auxiliary, for example, the surface of the active material is coated with a silane compound such as alkoxysilane, and a conductive auxiliary is attached to the surface. And heat treatment is further performed.

The coating treatment with the alkoxysilane can be carried out using an alkoxysilane solution in which an alkoxysilane is added to water or alcohol. In particular,
A method of mixing and stirring the positive electrode active material and the alkoxysilane solution using a disperser or the like, and a method of spraying the alkoxysilane solution to coat the positive electrode active material are not particularly limited.

[0010] After the above-mentioned alkoxysilane is coated, a conductive auxiliary is further adhered uniformly. Specifically, a method of mixing a cathode active material coated with alkoxysilane and a conductive additive using a disperser such as a ball mill or a kneader is preferable, but the method is not particularly limited.

After the conductive auxiliary agent is attached, heat treatment is performed to stabilize the adhesion between the positive electrode active material and the conductive auxiliary agent, and the positive electrode in which the silane compound and the conductive auxiliary agent are adhered to the surface of the positive electrode active material. An active material can be obtained.

In the positive electrode active material of the present invention, since the conductive additive uniformly adheres to the surface of the active material, the conductivity is greatly improved.
The amount of the conductive additive added can be significantly reduced as compared with the related art. Therefore, it is possible to provide a non-aqueous secondary battery having excellent impedance characteristics while reducing the amount of the conductive additive added as compared with the related art.

When the volume resistivity of the positive electrode active material used in the present invention was measured, the volume resistivity was lower than that of the untreated positive electrode active material, and the lithium ion secondary battery using this active material had excellent impedance. Properties were shown.
This is presumed to be due to the excellent electron conductivity because the conductive auxiliary uniformly covers the surface of the positive electrode active material with a strong bond together with the silane compound. further,
It is considered that in the coating film, the contact area between the positive electrode active materials was reduced, and the contact area between the active material and the conductive auxiliary agent or the conductive auxiliary agent was greatly increased, whereby an excellent conductive path was formed. . Further, even under severe conditions such as repeated charge / discharge and storage tests, the conductive path can be stably present, so that excellent impedance characteristics are maintained.

The above alkoxysilane is represented by the formula (1)
The compound represented by is preferably used.

X _a -Si-R _4-a Formula (1) (where X is -C ₆ H ₅ ,-(CH ₃ ) ₂ CHCH ₂ , -C _m
CH _{2m + 1} , wherein R is —OCH ₃ , —OC ₂ H
_5, a -OC ₃ H _7, a: 0~3 integer, m: 1 to 1
It is an integer of 2. )

Specifically, methyltriethoxysilane,
Dimethyldiethoxysilane, tetraethoxysilane, phenyltriethoxysilane, diphenyldiethoxysilane, methyltrimethoxysilane, tetramethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane, methyltributoxy Examples include silane, dimethylbutoxysilane, tetraethoxysilane, decyltributoxysilane, and the like.

When a carbon material is used as the conductive additive,
In consideration of the adsorption effect thereon, methyltriethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, isobutyltrimethoxysilane, and phenyltrimethoxysilane are preferred, and methyltriethoxysilane and methyltrimethoxysilane are most preferred.

The coating amount of the silane compound depends on the amount of the positive electrode active material 10
The amount is preferably from 0.01 to 5 parts by mass with respect to 0 parts by mass. It is more preferably from 0.03 parts by mass to 3 parts by mass, and still more preferably from 0.05 parts by mass to 1 part by mass.

When the coating amount is less than 0.01 parts by mass, the coating amount of the silane compound on the positive electrode active material is too small, and it is difficult to sufficiently adhere the conductive assistant. When measured using a volume resistivity measurement device,
It turned out that 0.01 mass parts or more is a preferable coating amount.

On the other hand, if the coating amount exceeds 5 parts by mass,
In addition to the decrease in the volume ratio of the positive electrode active material, the charge / discharge reaction may be inhibited by excessive coating with the silane compound, and the capacity may be reduced.

As the conductive aid, carbon materials and metal powders can be used. Commercially available carbon materials such as graphite materials, furnace black, channel black, acetylene black and Ketjen black can be preferably used. . Specifically, $ 3050, $ 3030,
# 3040, # 3230, # 3350, # 52, # 5
0, $ 47, $ 45, $ 33, $ 32, $ 30, $ 65
0, $ 750, $ 850, $ 2200, $ 2600 (Mitsubishi Chemical) Vulcan XC-72, Vulcan XC-72R,
Black Pearls 3500, Black Pearls 120, Black Pe
arls800, Black Pearls1400 (Cabot) Denka Black (Electrical Chemical Industry), Ketjen Black EC, Ketjen Black EC600JD (Ketjen Black International), Raven 14, Raven 22, Rave
n2000 (Columbian) and the like, but there is no need to limit to these.

The amount of the conductive additive attached to the alkoxysilane-coated positive electrode active material is preferably from 0.1 to 10 parts by mass based on 100 parts by mass of the positive electrode active material. More preferably, it is 0.5 to 5 parts by mass.

If the addition amount is less than 0.1 part by mass, the amount of the silane compound attached to the coated positive electrode active material is insufficient, so that the volume resistivity is high and the effect of reducing the impedance in the battery is not very effective.

If the addition amount exceeds 10 parts by mass, the volume ratio of the positive electrode active material is reduced and the capacity is reduced.
A range of 0 parts by mass or less is preferred.

The particle size of the conductive additive ranges from 0.005 μm to 1
μm is preferred. More preferred is 0.01 μm to 0.5 μm, and even more preferred is 0.015 μm to 0.1 μm.
1 μm.

A conductive auxiliary agent having a particle size of less than 0.005 μm has a very small particle size, so that it is poor in handling and productivity. Further, when the conductive auxiliary agent having a particle size exceeding 1 μm is coated with the positive electrode active material, the contact area between the conductive auxiliary agents is reduced, and the effect of increasing the electron conductivity is hardly obtained.

As the positive electrode active material, a lithium-containing double oxide represented by LiCoO ₂ , LiMn ₂ O ₄ or LiNiO ₂ is preferably used. The effect of the present invention is not particularly limited as long as it is a known positive electrode active material, but the effect of the present invention is more remarkable when a positive electrode active material having a BET specific surface area of 0.5 m ² / g or more is used. appear. In the positive electrode active material having a high specific surface area, the contact area between the active materials in the positive electrode increases, so that the impedance characteristics are likely to be reduced. However, according to the present invention, when the conductive additive uniformly adheres to the surface of the positive electrode active material, it becomes a positive electrode active material having excellent electron conductivity, and the contact area between the conductive additives increases, and the impedance characteristics are excellent. It is possible to provide a non-aqueous secondary battery.

The positive electrode active material is prepared by using a mortar, a crusher, or the like.
It is desirable to perform dry pulverization mechanically. This is because the aggregate of the positive electrode active material is loosened so that the silane compound can be more easily coated before the silane compound coating treatment with the alkoxysilane. The pulverizing treatment of the positive electrode active material is not limited to a mortar or a pulverizer, and is not particularly limited as long as the aggregate is broken up by mechanical dispersion and pulverization.

As described above, the method of coating the surfaces of the particles of the positive electrode active material with alkoxysilane is to mechanically mix and stir the positive electrode active material and the alkoxysilane, or spray the alkoxysilane onto the positive electrode active material. What is necessary is just to mix and stir mechanically while doing. The coating method is not limited to these. Thereby, almost all of the added alkoxysilane is coated on the particle surface of the positive electrode active material.

As a device for stirring the positive electrode active material and the alkoxysilane, a device capable of applying a shearing force to the powder layer is preferable, and in particular, a device capable of simultaneously performing shearing, spatula and compression, for example, , Wheel type kneader,
A ball-type kneader, a blade-type kneader, and a roll-type kneader can be used. In practicing the present invention, a ball-type kneader can be used more effectively.

Specific examples of the wheel type kneader include a multi-mul, a Stotts mill, a wet pan mill, a conner mill, a ring muller and the like. Examples of the ball type kneader include a planetary ball mill and a sand mill. Specific examples of the blade type kneader include a Henschel mixer, a planetary mixer, a Nauta mixer, and a Brabender. Specific examples of the roll-type kneader include an extruder.

The kneading machine used for kneading is not particularly limited to these. The conditions of the mixing and stirring are desirably to make the coating of the silane compound on the positive electrode active material more uniform. The number of revolutions, dispersion time, dispersion bead diameter, etc. are adjusted, and the dispersion time is preferably about 30 to 60 minutes.

The alkoxysilane may be used as it is without diluting it in a solvent, but it is preferable to use an alkoxysilane solution diluted in alcohol or water. This is because in order to coat the cathode active material surface more uniformly,
This is because it is preferable to stir the lower viscosity solution.

The dispersion time for coating the conductive additive is as follows:
It is preferably performed for about 30 minutes to 3 hours, depending on the specific surface area and particle size of the conductive additive.

After attaching the conductive auxiliary to the alkoxysilane-coated positive electrode active material, drying and heat treatment are performed. The heat treatment temperature is preferably from 50 ° C to 150 ° C.
The drying time is preferably from about 10 minutes to about 5 hours. By this heat treatment step, the alkoxysilane becomes a stable silane compound, and a positive electrode active material in which a conductive additive is more closely adhered to the surface of the positive electrode active material is obtained.

In the present invention, other components such as the negative electrode active material and the electrolytic solution are not particularly limited, and conventionally known components can be used.

[0037]

EXAMPLES Hereinafter, the effects of the present invention will be described specifically with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

(Example) Treatment of Positive Electrode Active Material 300 g of lithium cobalt oxide, which is a positive electrode active material, was placed in a zirconia pot and rotated at 100 rpm by a planetary ball mill.
Aggregates were loosened at rpm for 10 minutes. At this time,
Zirconia beads having a diameter of 1 mm were put into ポ ッ ト of the pot volume. Hereinafter, dispersion was performed using the same beads.

Then, 15 g of methyltriethoxysilane
Was mixed and diluted with 150 ml of an ethanol solution, and then this methyltriethoxysilane solution was put into a zirconia pot, and stirred at 250 rpm for 30 minutes by a planetary ball mill to coat the positive electrode active material with alkoxysilane.

6 g of carbon black # 3050 (manufactured by Mitsubishi Chemical Corporation) as a conductive aid was added to the positive electrode active material dispersion.
The carbon black was added and lightly immersed in the solution, and then mixed and stirred at 300 rpm for 3 hours using a planetary ball mill to attach 2 parts by mass of carbon black to 100 parts by mass of the active material on the surface of the positive electrode active material. I let it.

After filtering the obtained positive electrode active material dispersion,
Heat treatment was performed at 120 ° C. for 2 hours using a dryer to evaporate residual moisture, ethanol, and the like. This allows
A positive electrode active material whose surface was uniformly coated with a silane compound and a conductive additive was obtained. The positive electrode active material was subjected to elemental analysis to determine the coating amount of the silane compound. The amount was 0.8 parts by mass in terms of Si with respect to 100 parts by mass of the positive electrode active material.

[0042]Preparation of positive electrode 97 parts by mass of the positive electrode active material after the coating treatment was used as a binder.
3 parts by mass of polyvinylidene fluoride (PVdF)
Use N-methylpyrrolidone (NMP) as solvent
And mix and disperse with a planetary mixer.
An extreme paint was prepared. Put the above positive electrode paint on a mesh 80 mesh
After passing through to remove coarse particles, a 20 μm thick aluminum
Coating on both sides of the positive electrode current collector made of
After drying, a positive electrode active material-containing coating film was formed. Per unit area
Electrode weight is 25.0 mg / cm ^TwoMet. However
The positive electrode made from this is shared with the negative electrode, separator, etc.
When the electrode body has a wound structure, after drying this band-shaped body,
It was compression molded to a thickness of 170 μm. After cutting, aluminum
The lead member was welded to produce a sheet-shaped positive electrode.

Preparation of Negative Electrode As the negative electrode active material, the distance between the 002 planes (d002):
0.337 nm, crystallite size in the c-axis direction (Lc):
180 parts by mass of a graphite-based carbon material having characteristics of 95 nm, average particle diameter: 10 μm, and purity of 99.9% or more, 14 parts by mass of polyvinylidene fluoride and N-methylpyrrolidone 19
It was mixed with a solution dissolved in 0 parts by mass to prepare a negative electrode active material-containing paste. This negative electrode active material-containing paste was uniformly applied to both surfaces of a negative electrode current collector made of a 10-μm-thick strip-shaped copper foil, and dried to form a negative electrode active material-containing coating film. The electrode weight per unit area was 11.8 mg / cm ² . After drying, the band was compression-molded to a thickness of 167 μm and cut into a predetermined size. Next, the lead body made of nickel was welded to produce a strip-shaped negative electrode.

Preparation of Electrolyte Solution LiPF ₆ was added to a mixed solvent obtained by mixing methyl ethyl carbonate and ethylene carbonate at a volume ratio of 2: 1.
It was dissolved to a concentration of 2 mol / l to prepare a non-aqueous electrolyte (electrolyte solution).

Preparation of Secondary Battery After the above-described positive electrode and negative electrode were dried, the positive electrode and the negative electrode were spirally wound through a separator made of a microporous polyethylene film having a thickness of 25 μm to obtain an electrode body having a wound structure. . This is inserted into a bag-shaped aluminum laminated film, and after the above-mentioned electrolyte solution is injected, vacuum sealing is performed, and then left for 3 hours at room temperature in this state, so that the cathode, the anode and the separator are sufficiently impregnated with the electrolyte solution. A non-aqueous secondary battery was manufactured.

Comparative Example 1 A positive electrode was prepared in the same manner as in the Example except that 95 parts by mass of lithium cobalt oxide was used instead of the positive electrode active material coated with a silane compound and a conductive auxiliary. A non-aqueous secondary battery was manufactured.

With respect to each of the non-aqueous secondary batteries of the above Examples and Comparative Examples, changes in discharge capacity and impedance when charging and discharging were repeated were measured. Similarly, changes in the discharge capacity and impedance after the storage test were measured. Table 1 shows the results.

The discharge capacity was defined as the capacity at which a 1 C current limiting circuit was provided, charging was performed at a constant voltage of 4.2 V, and discharging was performed until the battery voltage dropped to 3 V. The change in capacity due to the repetition of charge and discharge is 300 cycles between the first cycle
Evaluation was made by measuring the discharge capacity at the cycle.
The storage characteristics were evaluated by measuring the discharge capacity after storage at 60 ° C. for one week.

In Table 1, the discharge capacity in the first cycle of the battery of the comparative example was defined as 100, and the relative value with respect to the discharge capacity was shown.

The impedance was measured at 1 kHz with an LCR meter under the same conditions as the battery capacity, and expressed as a relative value with the first cycle impedance of the comparative example being 100.

[0051]

[Table 1]

The non-aqueous secondary battery of the embodiment using lithium cobaltate whose surface is coated with a silane compound and a conductive auxiliary has a higher discharge capacity in the first cycle than the battery of the comparative example, and has a higher charge / discharge capacity. Repeat the cycle, or
Even after storage at high temperature, there was little decrease in discharge capacity.
In addition, it can be seen that the batteries of the examples have lower impedance than the comparative examples, and there is almost no increase in impedance after 300 cycles and after high-temperature storage.

[0053]

The use of the positive electrode active material coated with the silane compound and the conductive additive for the positive electrode of a non-aqueous secondary battery can improve the discharge capacity and greatly improve the impedance characteristics.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazushi Miyata 1-88 Ushitora, Ibaraki-shi, Osaka F-term in Hitachi Maxell Co., Ltd. 5H029 AJ05 AK03 AL07 AM03 AM05 AM07 CJ22 DJ08 EJ04 EJ11 HJ02 5H050 AA07 AA10 AA12 BA17 CA08 CA09 CB08 DA02 DA10 EA09 EA22 GA22 HA01

Claims (3)

[Claims] 1. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein a positive electrode active material present in the positive electrode is coated with a silane compound and a conductive additive. 2. The non-aqueous solution according to claim 1, wherein the coating amount of the silane compound on the positive electrode active material is 0.01 to 5 parts by mass in terms of Si based on 100 parts by mass of the positive electrode active material. Secondary battery. 3. The positive electrode active material according to claim 1, wherein the coating amount of the conductive auxiliary with respect to the positive electrode active material is 0.1 to 10 parts by mass based on 100 parts by mass of the positive electrode active material. Non-aqueous secondary battery.